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United States Department of Agriculture

Agricultural Research Service


Location: Children's Nutrition Research Center

Title: Rate of phenylalanine hydroxylation in healthy school-aged children

item Hsu, Jean
item Jahoor, Farook
item Butte, Nancy
item Heird, William

Submitted to: Pediatric Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2010
Publication Date: 4/1/2011
Citation: Hsu, J.W., Jahoor, F., Butte, N.F., Heird, W.C. 2011. Rate of phenylalanine hydroxylation in healthy school-aged children. Pediatric Research. 69(4):341-346.

Interpretive Summary: Our body contains many kinds of proteins which are made from 20 amino acids in different combinations. We get these 20 amino acids from the protein in our diet, but our body can also make 11 of them. The other 9 amino acids cannot be made by our body and is obtained solely from our diet. If our body does not get enough amino acids, this can lead to growth failure. Phenylalanine is an amino acid that our body cannot make. Tyrosine is another amino acid that our body can make from phenylalanine. We were interested to find out whether school-aged children can make all the tyrosine they need for normal growth from phenylalanine. Compared to adults, children made less tyrosine when they ate a diet without tyrosine. We believe that healthy children are able to make some tyrosine, but they also need some tyrosine from their diet in order to fulfill their total needs for normal growth.

Technical Abstract: AB Hydroxylation of phenylalanine to tyrosine is the first and rate-limiting step in phenylalanine catabolism. Currently, there are data on the rate of phenylalanine hydroxylation in infants and adults but not in healthy children. Thus, the aim of the study reported here was to measure the rate of phenylalanine hydroxylation and oxidation in healthy school-aged children both when receiving diets with and without tyrosine. In addition, hydroxylation rates calculated from the isotopic enrichments of amino acids in plasma and in very LDL apoB-100 were compared. Eight healthy 6- to 10-y-old children were studied while receiving a control and again while receiving a tyrosine-free diet. Phenylalanine flux, hydroxylation, and oxidation were determined by a standard tracer protocol using oral administration of 13C-phenylalanine and 2H2-tyrosine for 6 h. Phenylalanine hydroxylation rate of children fed a diet devoid of tyrosine was greater than that of children fed a diet containing tyrosine (40.25 +/- 5.48 versus 29.55 +/- 5.35 umol/kg/h; p < 0.01). Phenylalanine oxidation was not different from phenylalanine hydroxylation regardless of dietary tyrosine intake, suggesting that phenylalanine converted to tyrosine was mainly oxidized. In conclusion, healthy children are capable of converting phenylalanine to tyrosine, but the need for tyrosine cannot be met by providing extra phenylalanine.

Last Modified: 06/22/2017
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